1. Field of the Invention
This invention relates to a potentiometer for use in a rotary positioning sensor and other type of positioning sensors.
2. Description of the Related Art
A conventional potentiometer for detecting a rotary or linear position is shown in FIG. 9.
This potentiometer includes a resistive element 40 formed between a ground terminal 41 and a power terminal 42. The resistive element 40 has a print or a conductive plastic substrate. A sliding potion 44 is slidably mounted on the resistive element 40. A switch pattern (not shown) is formed on the substrate for detecting a specific position. Numeral 43 designates an output terminal of the potentiometer. Detected output is obtained at the output terminal 43 in response to the position of sliding portion 44. Reference output voltage is set by the output which is defined by the position of switch pattern. The output characteristic of this conventional device is shown in FIGS. 10 and 11 wherein the relationship between the position of a sliding portion 44 and the output voltage is shown.
FIG. 10 shows output characteristic before the adjustment of offset and FIG. 11 shows output characteristic after the adjustment of offset is made.
In FIG. 10, two dotted lines show the theoretic output OT and the output before the adjustment is shown at the curved line OF which is substantially smaller than the theoretic output in the entire range. In the drawing, position R1 shows the reference theoretic position and ratio R2 shows the output at the reference position R1. The position R3 is offset from the reference theoretic position R1 with the amount of .delta.1. In this device, the housing is moved with the amount of .delta.1 in order to compensate the offset.
FIG. 11 is one example of output characteristic showing a relation between a position of the sliding portion 44 and an output voltage ratio (%) after adjustment is made.
In FIG. 11, the position R4 is a reference position of the sliding portion 44 after the offset adjustment is made. Ratio R5 is an output at the reference position R4. Line OB shows an output characteristic after the offset adjustment is made.
In the above-described potentiometer, the position is detected based on the reference voltage. However, the resistive element 40 is worn away because the sliding portion 44 continuously slides on the resistive element 40.
As the resistive element 40 is worn away, the potentiometer fluctuates the output. The position where the reference output is generated on the sliding portion 44 is changed due to the worn resistive element 40.
In the above-described potentiometer, a reliable and simultaneous control cannot be obtained when a detected output is supplied to a control system, because the output which is detected by the potentiometer is fluctuated.
In the conventional potentiometer, a position of the housing has to be adjusted with respect to an object to be detected in order to compensate the offset.
In the above-described potentiometer, a detected output tends to be different from a theoretic output because a length between a ground terminal 41 and a power terminal 42 is relatively long.